1. Field of the Invention
The present invention relates to an optical scanning device and an image forming apparatus.
2. Description of the Related Art
In an image forming apparatus employing the Carlson process, a beam deflected by a deflecting unit such as a polygon mirror scans a surface of a rotating photosensitive drum, so that a static latent image is formed on the surface. The electrostatic latent image is developed into a visible toner image with toner. The toner image is transferred onto a recording medium and fixed thereon. Such image forming apparatuses are often employed for simple printing as an on-demand printing system, and there is an increasing demand for a system that forms (outputs) a high-density image at high speed.
By increasing the rotation rate of the polygon mirror and the photosensitive drum, it is possible to output an image at high speed. However, an increase in rotation rate of the polygon mirror increases noise and vibration from a driving system in addition to power consumption, which decreases the durability of the image forming apparatus. There is a trade-off relationship between increasing of printing speed and image density, and the increase in the rotation rate of the polygon mirror decreases image quality.
To print a high-density image at high speed, Japanese Patent Application Laid-open Nos. 2005-250319 and 2004-287292 disclose a method in which a multibeam light source emitting a plurality of beams is used and the beams scan a surface of a photosensitive drum simultaneously.
However, because the beams emitted from the multibeam light source are incident on a deflecting surface of a deflecting unit at different angles, the beams tend to be shaded on the deflecting surface. To prevent the beams from being shaded, the beams needs to be incident on the deflecting surface at a small angle or the area of a surface to be scanned needs to be limited (i.e., the writing area needs to be small).